1. Field of the Invention
The present invention relates generally to semiconductor circuits and, more particularly, to capacitive structures for suppressing inductive noise in high performance integrated circuit applications.
2. Description of the Related Art
The continuing trend in modem integrated circuit design is to decrease the physical size of a chip, increase circuit layout density, and increase operating speeds. Although designers have been able to design very high frequency devices that operate at lower power supply voltages, a substantial increase in inductive noise due to a chip's package and printed circuit board (PCB) routing has also occurred. In the past, off-chip discrete capacitor components have been attached directly onto a chip's package or the PCB in order to capacitively shunt the ever increasing inductive noise. Although this was worked with some success in the past, as devices continue to be improved to operate at faster speeds and lower power supply voltages, the allowable noise margin for the inductive noise has also continued to decrease. That is, as devices become faster and faster, a device may fail to operate properly if the inductive noise rises above predetermined allowable noise margins, which necessarily decrease as speeds increase.
FIG. 1A is a simplified diagram of a PCB 100 having a packaged chip integrated onto the PCB 100. In this example, a chip 102 is integrated onto a package 104, which has a plurality of traces 106 that connect the chip 102 to the lead pins of the package 104. As mentioned above, a customary method of suppressing the inductive noise is to attach off-chip discrete capacitor components 108 or 112 on the package 104 or the PCB 100, respectively. Although this has worked in the past, the increased amount of inductive noise produced in devices having clock frequencies above about 200 MHz has been found to be too much for off-chip discrete components to handle. As a result, many high speed devices suffer in having noise margins that exceed the amount recommended for proper high performance operation.
For illustration purposes, FIG. 1B shows a graph 150 that plots allowable noise margins vs. clock frequencies. A plot line 152 illustrates that as clock frequencies increase, the allowable noise margins also decrease. For example, a point A shows that for a 0.5 micron technology device, the allowable noise margin is slightly below about 12% of the voltage source (i.e., Vdd) used. FIG. 1C provides a graph 160 that plots the allowable noise margins as a percentage of the voltage source, where a full rail Vdd is 100%.
Referring back to FIG. 1B, a point B illustrates that the allowable noise margin drops to about 10% of the voltage source used for a 0.35 micron technology device. A point C illustrates that the allowable noise margin drops again to about 8% of the voltage source used for a 0.25 micron technology device, and a point D illustrates that the allowable noise margin drops to about 6% of the voltage source used for a 0.18 micron technology device. FIG. 1C also plots the allowable percentage noise margin for a 0.25 micron technology device.
As can be appreciated, the faster the device gets, the smaller the allowable noise margin becomes, and because lower voltage sources are used for smaller technology devices, the actual voltage magnitude of the allowable noise margin also decreases more substantially. To illustrate this point, reference is drawn to FIG. 1D, where the exemplary micron technologies are compared with respect to the allowable noise margins (NMs), and the resulting voltage magnitudes.
For example, for a 0.5 micron technology device, the voltage source is 5V and the allowable noise margin is about 12%. This therefore produces a noise margin voltage magnitude of about 0.6V. This can then be compared to a 0.18 micron technology device which has a voltage source of 1.8V, and an allowable noise margin of about 6%. The resulting noise margin voltage magnitude will be about 0.1V. Thus, not only does the allowable noise margin decrease as device speed increases, but the voltage supply used in smaller micron technology devices also decreases.
Consequently, that faster and smaller the device, the less it will be able to handle even very small noise margins. Further yet, the faster the device becomes, even more inductive noise tends to be produced. To remedy this, some designers have begun to custom design on-chip capacitors into existing designs. Unfortunately, the design of custom on-chip capacitors has the disadvantage of requiring a substantial amount of chip area, which in some cases may be up to 10% or more additional chip area. Further yet, the design of custom on-chip capacitors also requires time consuming manual examination and modification of the photolithography masks used to design the multi-layered devices.
As can be appreciated, this type of custom design can be very costly, however, this expense has become necessary in order to appropriately suppress the growing levels of inductive noise produced in higher performing devices.
In view of the foregoing, there is a need for techniques that enable fast and efficient design and fabrication of on-chip capacitive structures for suppressing power supply inductive noise without impacting density or enlarging costly chip area